Attorney-General George Brandis says at least 20 Australians have been killed fighting alongside terrorist groups in Syria and Iraq, and warns that the Islamic State group is using Australians on the frontline as "cannon fodder, bombers and propaganda tools".

Senator Brandis said the number of Australians killed had risen in recent weeks and that Western recruits were being duped into thinking they were an important part of a religious crusade.

Around 70 Australians were still believed to be fighting in the Middle East while another 20 had returned home.

Victoria Police Chief Commissioner Ken Lay said would-be jihadists were "very likely" to be killed if they joined the conflict.

"You'd hope that mainly young men who are considering this understand that they're very likely to get killed if they do go over and fight," he told Fairfax Radio.

"I think it is a very important part of the message about discouraging young men going over to fight."

Mr Lay said police were concerned social media was being used to recruit young people to join the conflict

"Obviously there's attempts to get young people to go over to the Middle East and fight," he said.

"We're still watching it very closely and we're working with other police jurisdictions and other agencies to try and prevent that happening."

Among those fighting is Sydney man Mohammad Ali Baryalei, who has been accused of masterminding a plot to kill random members of the public in Sydney and Brisbane, and had recruited dozens of Australians to fight with extremist groups in Iraq and Syria.